PHYSIOLOGICAL CHEMISTRY 



441 



Microscopically they are seen to be very small octahedra, often 

 flattened along one axis, so that they appear like squares with diagonal 

 lines (hence called "envelope" crystals, Fig. 252). 



Acid urine from a person suffering from disease, or during the administra- 

 tion of certain drugs, may deposit : 



1. Cystin. This forms a deposit somewhat like that of urates in 

 appearance. 



Fio. 253. Stellar phosphate of calcium, x 500. 



Microscopically, however, it shows a distinct crystalline structure 

 consisting of hexagonal colourless plates or slabs (Fig. 250). When the 

 crystals are present the condition is called cystinuria (see p. 438). 



2. Leucin and Tyrosin. Though very rarely, these two bodies 

 sometimes occur in urine (e.g. in severe hepatic disease), where their 

 appearance is similar to that in a pancreatic digest (see Fig. 236). 



3. Hippuric Acid. This may appear in urine during the administra- 



